A new 3D luminescent Zn(ii)–organic framework containing a quinoline-2,6-dicarboxylate linker for the highly selective sensing of Fe(iii) ions

Abstract

A new 3D zinc–organic framework [Zn(QDA)]·0.5H₂O·0.7DMF (1, H₂QDA = quinoline-2,6-dicarboxylic acid, DMF = N,N-dimethylformamide) was synthesized under solvothermal conditions. The single crystal X-ray diffraction analysis reveals that the 3D framework structure of 1 has a PtS topology and contains Zn(II) ions having distorted square pyramidal geometry with ZnO₄N configuration. The phase purity of the bulk sample was characterized by X-ray powder diffraction (XRPD), thermogravimetric analysis (TGA) and Fourier transform infrared (FT-IR) spectroscopy. The as-synthesized sample (1) was activated by stirring with acetone for 24 h, followed by heating under vacuum for 24 h at 120 °C. The TGA experiment indicated that both 1 and its activated form (1′) are stable up to 380 °C. The crystalline structure of the compound was retained after immersion in water, 1 M HCl, acetic acid and NaOH (at pH = 10) solutions. Compound 1′ exhibited very quick fluorescence quenching response after the addition of Fe³⁺ solution. This quenching was not affected by the presence of other competitive metal cations. A very low detection limit of 9.2 ppb was observed for Fe³⁺ ions, which is among the lowest values documented in the literature for MOF based fluorescent probes. Both fluorescence resonance energy transfer (FRET) and photo-induced electron transfer (PET) processes play major roles in the selective detection of Fe³⁺ ions. The recyclability experiment suggested that 1′ can be used for the long-term detection of Fe³⁺ ions.